1. Field of the Invention
This invention relates to a dielectric ceramic composition and, more particularly, to a high permittivity dielectric ceramic composition.
2. Description of the Prior Art
So far, dielectric ceramic compositions of a barium titanate system have widely been put into practical use as ceramics of a high permittivity dielectric. However, when these BaTiO.sub.3 dielectric ceramics are applied to capacitors, the dielectric loss increases with increasing frequencies and the capacitance is considerably affected by a direct current bias electric field. For example, when a high DC voltage of 5 KV/mm is applied to the capacitor, the capacitance thereof is reduced by 30 to 50%.
On the other hand, many investigations of SrTiO.sub.3 systems have been reported in various literatures such as, for example, in "DIELECTRIC PROPERTIES AND STRUCTURE OF PbTiO.sub.3 --SrTiO.sub.3 --Bi.sub.2/3 TiO.sub.3 SOLID SOLUTIONS" in ENGLISH TRANSLATION of the BULLETIN of the ACADEMY of SCIENCES OF THE USSR, PHYSICAL SERIES, Vol. 24. No. 11, p1383-1386, by V. Ya. Fritsberg, E. Zh. Freidenfel'd & Ya. Ya. Kruchan, Japanese patent publication No. 41880/1977, and Japanese patent publication No. 31280/1978. The first and second references disclose a dielectric ceramic composition of a PbTiO.sub.3 --SrTiO.sub.3 --Bi.sub.2 O.sub.3 --TiO.sub.2 system and the third reference discloses a dielectric ceramic composition of a SrTiO.sub.3 --PbTiO.sub.3 --Bi.sub.2 O.sub.3 --TiO.sub.2 --CaTiO.sub.3 system.
The ceramics of these systems possess a high dielectric constant of not less than 500 and is low in a change of capacitance with bias electric field as compared with those of the BaTiO.sub.3 system, so that they have been put into practical use.
However, these dielectric ceramics also have various problems awaiting a solution in their characteristics and in industrial mass production. For example, a temperature change rate of dielectric constant is large. In addition, since the ceramic compositions of these systems must be fired at a high temperature ranging from 1220.degree. to 1320.degree. C. to obtain optimum dielectric characteristics, it is difficult to avoid evaporation of easily evaporable PbO and Bi.sub.2 O.sub.3 during sintering without closely controlling the sintering atmosphere of PbO and Bi.sub.2 O.sub.3.